This invention is directed to particle counting methods and apparatuses which provide a statistic correction to a detected train of particle derived count pulses, such that effective random coincidence inaccuracies of count does not induce ultimate counting error.
The particle counting methods and apparatuses concerned employ particle sensing zones in which more than one particle might reside at any one time and thereby randomly generate a coincidence condition. This invention particularly is directed to, but not limited to the determination of nonelectrical properties, such as size and count of microscopic particles, by measuring electrical properties (Patent Office class 324-71NE).
Now well known in the art of electronic particle counting and analyzing is apparatus marketed primarily under the trademark "Coulter Counter". Such apparatus and portions thereof are disclosed in several U.S. patents, for example U.S. Pat. Nos. 2,656,508; 2,985,830; and 3,259,842 (each in class 324-71). A significantly important portion of such Coulter type of apparatus is the minute scanning aperture or scanning ambit or sensing zone relative to or through which pass and are detected single particles at a rate often well in excess of one thousand per second. Because of the physical parameters of the scanning aperture, and particle concentration, there frequently results the coincidence of two particles in the scanning ambit. As a result, there is effectively detected and counted only one particle, not two.
Although such primary form of coincidence loss of count is random in time, it follows a statistically ascertainable form, from which curves, tables, and formulae are obtainable. A relatively simple one of such formulae is: N' = K N.sup.2 in which N' = the total number of coincidences, i.e., the required addend; K = a constant which relates primarily to the physical parameters of the scanning elements of the apparatus and N = the detected number of particles, the augend. Accordingly, the true or corrected count N.sub.O will equal the sum of N+N'.
Heretofore, the operator of a "Coulter Counter" would obtain the augend count by analysis of a suspension of particles and then would refer to a coincidence correction chart which presented the proper corrected count for a very large selection of augends.
Although the use of charts provides an accurate result, it is both time consuming and prohibits the fully automatic recording and processing of the corrected counts. Also, of course, the accumulating count during analysis is uncorrected. Different charts must be used with apertures of different sizes.
The use of analog, non-linear meters and/or elements in the output stage of a "Coulter Counter" has also been accomplished with limited success; however, in may uses a direct reading digitalized output is greatly to be preferred.
A recently developed, automatic digitalized system and method for coincidence correction, that yields a continuously corrected true count N, is the subject of U.S. Pat. No. 3,626,164. According to the therein described invention, various amounts of the addends are periodically generated by a somewhat complex arrangement of counters and are periodically applied to the continuously accumlating augend count of the particles to yield the true count. Although such method and apparatus provide a distinct advantage over the prior art, they possess the basic limitation of being programmed to a specific correction factor constant K, which itself is tied to the physical parameters of the detecting apparatus, such as the size and volume of the detecting aperture. Hence, changes, such as in aperture size, reqire program changes in the correcting apparatus. Additionally, this somewhat complex arrangement presents a purchase and maintainence cost which must be considered from a commercial sense.
Considerable research effort has been devoted to the phenomena of particle coincidence in a "Coulter Counter". A few of the many publications resulting from such consideration are next listed: Wales, M. and Wilson, J. N., Theory of Coincidence in Particle Counters, Review of Scient. Instruments, Vol. 32, Nr. 10, pp. 1132-1136, Oct. 1961; Princen, L. H. and Kwolek, W. F., Coincidence Corrections for Particle Size Determination with the Coulter Counter, Review of Scientific Instruments, Vol. 36, Nr. 5, pp. 646-653, May 1965; Strackee, J., Coincidence Loss in Bloodcounters, Medical and Biological Engineering, Vol. 4, pp. 97-99, 1966; Princen, L. H., Improved Determination of Calibration and Coincidence Correction Constants for Coulter Counters, Review of Scient. Instruments, Vol. 37, Nr. 10, pp. 1416-1418, Oct. 1966; Edmundson, I. C., Coincidence Error in Coulter Counter Particle Size Analysis, Nature, Vol. 212, pp. 1450-1452; and Bennert, W. and Hilbig, G., The Theory of the Coincidence Error for Digital Particle Size Analysis, Staub-Reinholt, Luft, Vol. 27, Nr. 4, April 1967.
Reduced to its simplest, the count error due to coincidence can be corrected by adding a fraction of the uncorrected or raw count N.sub.R. Such fraction can be defined as the "correction factor" F.sub.C, for which EQU (1) F.sub.C = KN.sub.R,
in which K is a very small value "scanning constant". Accordingly, the computation for the true count N.sub.0 is: EQU (2) N.sub.O = (1 + F.sub.C) N.sub.R.
experimentation has shown that the scanning constant K is very nearly the ratio of the critical volume C.V. to the sample volume S; thus, ##EQU1## in which the critical volume is that volume defined by the scanning aperture of a Coulter type particle analyzer. Unfortunately, the critical volume is not a fixed amount for any one aperture, and certainly is not the same under varying input conditions. One of the variables that goes into the determination of the critical volume is the electronic field in the immediate vicinity outside of the scanning aperture, which must be included in what has been termed the "scanning ambit" of the particle detector. Such varying electronic field is discussed in U.S. Pat. No. 3,668,531.
Assuming that K is ascertainable, then from equations (1) and (2), EQU (4) N.sub.O = (1 + KN.sub.R) N.sub.R
or EQU (5) N.sub.O = N.sub.R + KN.sub.R.sup.2.
unfortunately, due to the extremely small size of most scanning apertures, K has not been easy to isolate with reliability and reproducibility for various input parameters.
Although the Coulter type of particle analyzer, with its aperture form of scanning or sensing zone, is specifically described herein, other forms of particle analyzers, such as those operating with light or acoustic energy and having optical or acoustic sensing zones are encompassed by the invention herein, to the extent that these other types of particle analyzers are subject to particle coincidence in their sensing zones.